The battery is powering your preamp (the knobs that cut and boost different frequencies). Here are the possible pickup and preamp configs:
passive pickups and a passive preamp (a Fender P or J for example)
passive pickups and an active preamp (Musicman Stingray and your bass are examples)
active pickups and active preamp (American and Czech Spectors.. most Modulus's I think)
or even active pickups and a passive preamp (though I don't know of any production bass with a setup like this).
Active means battery powered. A passive preamp can only cut treble (as far as I know), but an active preamp can cut or boost many bands of frequencies. I don't feel that I know the ins and outs of passive/active pickups enough to comment on them.
